1. Technical Field
The field of the invention relates to data processing and more particularly relates to a method and apparatus for linking SNA data processing equipment over a packet switched communications network.
2. Background Art
In 1974, IBM's System Network Architecture (SNA) significantly advanced the state of the art in teleprocessing software systems. E. H. Sussenguth, "Systems Network Architecture: A Perspective," Conference Proceedings, 1978 International Conference on Computer Communications, Kyoto, Japan, 1978, pp. 353-358; D. Doll, "IBM Strengthens its Architecture," Data Communications 8, 56-67 (1979). SNA provides a unified design for the functions and structure of data communications products. Prior to the introduction of SNA, teleprocessing networks had many problems: Terminals were often dedicated to the use of a single application, numerous and diverse line-control procedures and terminal types were ingrained into the support programs, application programs, and network operations; and multiple access methods were in common use, thwarting any attempt to share resources among applications. Each of these problems made it difficult to expand existing applications or to add new ones. SNA was introduced to solve these problems and to make teleprocessing applications easier to install, operate, and expand.
SNA also had its roots in the hardware technological advances of the early 1970s. At that time, it became economically possible to incorporate a small processor into the design of many terminals.
Prior to such microcomputers, a terminal was commanded directly by its host computer. For example, each keystroke produced an input character transmitted independently at the rate of generation; and each output character was sent at a rate not exceeding that of the printer.
With the new microcomputer-based designs, the processor within the terminal handles many functions independently of the host, and the transmissions between host and terminal are complete messages sent at high speed. This reduces the processing power required at the host and/or allows more terminals for a host of the same size. A more important change, however, was in system structure. No longer is a tight coupling between terminal and host needed; device control now can be placed at or near the end terminal and not in the host. Thus, system commands, protocols, and procedures designed for tight coupling are no longer required; instead, a new set specifically designed for distributed processing is required.
Just as the processor in the terminal now handles device control, it also readily becomes an application processor. From a system standpoint, the application may now be performed in any several places within a network--at the host, at a controller, or even at the terminal itself.
This is a new structure that essentially did not exist before 1974 and definitions for the control of such a system were needed. The advent of distributed processing, then whether for device control or distributed application processing, was the fundamental technical rationale behind the creation of SNA.
From an architectural point of view, SNA is a top-down structured design composed of layers. R. J. Cypser, Communications Architecture for Distributed Systems, Addison-Wesley Publishing Co., Reading, MA, 1978; SNA Technical Overview, Order No. GC30-3073, available through IBM branch offices. The lowest layer, data link control, directly manages physical resource--the transmission facilities that connect nodes. Successive layers provide additional services. For example, the path control layer provides a routing service so that its users are unaware of the physical topology of the network, and some nodes contain a control point that controls the nodes (e.g., terminals and controllers) and lines in their own portions of the network. Other layers provide services to applications; these can include transparent access to local or remote resources, mapping of data streams to and from application data structures (also called presentation services), access to other local or remote programs, management of buffer commitments, and encryption of data before transmission and decryption upon receipt.
In SNA, a network addressable unit (NAU) is a location in the SNA network that supports one or more ports for communication via the network. Each NAU has a network address.
SNA defines three types of NAU.
1. System Services Control Point (SSCP): A special-purpose NAU used for network management. An SNA network can have one or more SSCPs, each of which manages a portion of the network. The function of the SSCP is the general management of a control domain, such as bringing up the network, helping to establish logical connections between other NAUs, and helping in recovery and maintenance, when necessary. It also provides the interface to the network operator services for that domain.
2. Physical Unit (PU): An NAU that acts as a companion to the SSCP in SNA network configuration management. Each node that has been defined to an SSCP has at least one PU. The PU provides a location for configuration-related services which must be performed at a particular node. An SSCP and PUs together control the network configuration and the data-transportation resources provided by the nodes in the domain of the SSCP.
3. Logical Unit (LU): An NAU that provides windows or ports through which the end-user accesses the SNA network. The LU is also the port through which an end-user accesses SSCP-provided services to help in establishing logical connections between LUs. The LU may support communication between end-users (or LUs) by editing or transforming the requests, grouping requests, correlating responses with requests, and otherwise bridging from the environment of the end-user.
In SNA, four examples of nodes, hosts, communications controllers, cluster controllers, and terminal nodes, are designated as types T5, T4, T2 and T1, respectively. The architectural distinctions among them are in the layers and function subsets that are used for each type. These type numbers correspond to each node's PU-type (PU5, PU4, PU2, and PU1), which denotes the capabilities in the lower layers, particularly in data link control and path control.